Microvascular Beds - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 179

unit seen in skeletal muscle cells. The size of vascular smooth muscle cells is also smaller
than skeletal muscles. They typically range from 1 to 5

μ

m in diameter and approximately

μ

100 m in length.

Neighboring vascular smooth cells form one structural unit which contract or dilate as
one large muscle “fiber.” The cell membranes of these cells are connected to each other via
cadherin junctions (similar to endothelial cells) and gap junctions. This allows for the uni-
form force generation across the smooth muscle cells. Gap junctions are also present
within the sheet of smooth muscle cells to electrically couple neighboring cells. Gap junc-
tions allow for the free movement of ions between cells. Similar to the cardiac muscle cells,
this allows for the rapid transmittance of the action potential between cells. Interestingly,
there is a second group of specialized gap junctions that allow for the free communication
between endothelial cells and smooth muscle cells.
Briefly, there are many similarities between smooth muscle and skeletal muscle contrac-
tion. However, within smooth muscle cells, actin and myosin filaments are able to interact
without the troponin complex, which regulates skeletal muscle contraction. Smooth muscle
cell contraction is however regulated by calcium and adenosine triphosphate (ATP) con-
centration in a slightly different way than skeletal muscle. Also, instead of a sarcomere
sub-unit, smooth muscle cells have a more disorganized structure. Actin filaments are
directly connected to dense bodies, which are typically located within the cell membrane
of the smooth muscle cell (some dense bodies are located sporadically throughout the
cytoplasm). Myosin filaments are randomly dispersed throughout the cytoplasm, poten-
tially coming in contact with the actin filaments ( Figure 6.3 ). Cell membrane-associated
dense bodies of one smooth muscle cell can adhere to cell membrane-associated dense

FIGURE 6.3 Structure of a smooth muscle cell, with an inset show-
ing the relationship between actin, myosin, and the dense bodies.
Adjacent smooth muscle cells are connected by dense bodies, which are
also interspersed throughout the cells cytoplasm. The actin and the
myosin have a much more random orientation than skeletal or cardiac
muscle cells. Adapted from Guyton and Hall (2000).

Dense
bodies

Myosin

Actin